ARM: dts: am43xx-epos-evm: Don't use read/write wait monitoring

NAND uses wait pin only to indicate device readiness after
a block/page operation. It is not use to extend individual
read/write cycle and so read/write wait pin monitoring must
be disabled for NAND.

Add gpmc wait pin information as the NAND uses wait pin 0
for device ready indication.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Pekon Gupta <pekon@pek-sem.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>